The present invention relates to a method for drying or heat-treating products or substances, namely food products. Further it relates to an apparatus for drying or heat-treating products, namely food products. Finally, the invention relates to a dried food product.
To accelerate drying and for a more gentle treatment, heat-treating substances, such as agricultural products for example, is often done at a reduced pressure in a pressure-tight treatment chamber in which a heating source is arranged. Liquid products are usually pumped into the treatment chamber for treatment where they are evenly distributed on an endless conveyor belt which guides them past a heating source. In this arrangement the conveyor belt serves two functions, namely transporting, on the one hand, and, on the other, receiving and guiding the product to be treated. For this purpose the conveyor belt is usually configured troughed, for example upswept or provided with side sealing means. On completion of drying the treated product is scraped from the rear end of the belt and discharged via a dual gating system, for example. If the product to be treated is available as a solid or lumpy product a dual gating system is usually also provided for charging.
One such system is disclosed for example from DE 295 17 499 U1. This system comprises a treatment chamber in which microwave treatment and vacuum drying take place, the product to be treated being conveyed by an endless conveyor belt into the treatment chamber. Charging and discharging the treatment chamber is done via a dual gating system in exploiting the force of gravity. On impacting the conveyor belt the infalling product may be deformed and remain adhering to the conveyor belt. The product to be treated comes into contact with various components, such as with the dual gating system and the conveyor belt, for example.
In systems for heat-treating products at a pressure other than atmospheric pressure it is usually so that a change from one product to another product can only be made with a lot of trouble since many of the components are accessible only with restrictions, and need to be cleaned or replaced. Furthermore, satisfying hygiene code requirements is only possible at great expense.
The object of the present invention is to improve a method and an apparatus for drying or heat-treating products in a treatment chamber operating at a pressure other than atmospheric pressure. More particularly, the intention is to provide a method or apparatus with which drying or heat-treating of products is possible by ways and means which are hygienic, preferably without detrimenting taste and quality of the product to be treated as well as being cost-effective and low-wearing. Where food processing is concerned, it is furthermore the intention to make an end product possible having improved properties. Preferably, the method and the apparatus are intended to be suitable for drying banana chunks into banana chips, as starting product for a banana powder to be produced. Further banana chips and the banana powder to be produced therefrom are to be produced with as instant-type banana chips and powder. Furthermore, it is the intention to provide a transport receptacle suitable for implementing the method.
In accordance with the present invention there is provided a method of drying or heat-treating products in a treatment chamber, in which a pressure other than atmospheric pressure prevails, in which the products to be treated are charged in a transport receptacle. The transport receptacle is transported into the treatment chamber via a charging lock chamber in which the pressure is made equal to that existing in the treatment chamber. In the treatment chamber the product to be treated is guided past a heating source, preferably a microwave and/or infrared source by means of a main conveyor means arranged in the treatment chamber. This is followed by the discharge via a discharging lock chamber in which the pressure is made equal to another pressure, preferably to ambient air pressure. Preferably, to each charging and discharging lock chamber conveyor means are associated, which are independent of the main conveyor means in the treatment chamber.
In accordance with the invention the product to be treated does not come into contact directly with any of the conveyor means or any other components, thus effectively preventing soilage or contamination of the components difficult to access, for example conveyor belts in the treatment chamber. One advantage is that a change from one kind of product to another may be simply done without any detriment to the other product in each case, especially as regards taste, in the case of food products. Another advantage is that in accordance with the invention alternating operation is possible in products differing in provenance and quality may be processed quasi-continuously and without the one detrimenting the other.
Yet a further advantage is that the product to be treated or remainders thereof need not be removed mechanically from a conveyor belt in a vacuum chamber. This adds to the life of a conveyor belt arranged in the treatment chamber due to its reduced wear. Furthermore, the conveyor means are hardly exposed to transverse forces, due to the gentle continual transport, thus reducing the maintenance expense of the apparatus to advantage.
Still another salient advantage is that due to the use of transport receptacles for receiving the products to be treated freedom of design of the conveyor means arranged in the treatment chamber is permitted. In particular it does not need to be configured troughed or expansive any longer.
Preferably the transport receptacles after having passed through the treatment chamber are cleaned before being returned to a repeated processing cycle, thus ensuring hygienic operating conditions by simple ways and means. It is furthermore of advantage that the hygienic condition of the transport receptacles may be simply inspected and that bacteria, germs or spores are unable to multiply unchecked in the treatment chamber, for example in liquid films or product spillage in the treatment chamber. This is especially of advantage when processing or producing medicinal drugs subject to stringent hygiene code requirements. More particularly, in the case of highly-sensitive food products, such as for example banana chunks, berries or mushrooms, using preservatives may now be substantially reduced or even avoided altogether by simple ways and means.
Where allowed by the nature of the product concerned, more particularly in the case of slightly sticky or non-flowable products or substances, the transport receptacle is configured as a plate which need not be rimmed. Should, however, the nature of the product require such a rim, especially where liquid or grainy substances are involved, the transport receptacle is to be configured as a dish having a rim so that the product to be treated is unable to fall or flow off on the side, for example into the treatment chamber, to thus save cleaning and operating costs. Preferably the rim is configured on all sides so that no side spillage whatsoever exists which would otherwise result in loss of the product. The rim may be configured upright, vertical to the bottom of the receptacle or in another suitable way to effectively prevent loss of the product during treatment.
Even better hygienic conditions are possible when the product is placed on a net or grid or on a replaceable net frame arranged in the transport receptacle. By configuring the portion coming into contact with the product as an airy, lightweight net, cleaning it may be affected even simpler and more effectively. Furthermore, moisture emitted by the product during drying is able to escape practically unrestricted on all sides, which accelerates drying and thus helps in saving costs. Preferably a gap is provided between the net or the receiving surface and the transport receptacle to enable the emitted moisture to be removed even more expediently.
It is particularly in the case of processing medicinal drugs that sterile disposable transport receptacles or replaceable disposable nets may also be put to use, thus making the range of application of the apparatus or method even wider.
In one preferred embodiment to make more efficient use of the handling volume several such nets are stacked vertically in one and the same transport receptacle. For this purpose replaceable nets or net frames are provided. These nets or net frames may be identical in basic surface area. For stacking, the nets are preferably impaled on guide posts in the transport receptacle, cylindrical sleeves of suitable height serving as spacers between the individual nets. However, the nets or net frames may also differ in basic surface area, located on a stepped transport receptacle rim at various levels.
Where the consistency of the product permits, the bottom of a transport receptacle may be perforated so that the problem of any product remaining in the interior of the transport receptacle as well as contamination of the product is even further reduced. Such a transport receptacle also permits a convection drying in a downstream treatment zone without the product needing to be transfer-filled.
To prevent the product from being lost upwards on drying, for example by jumping out of place on being puffed, (i. e. a process by which the inner structure of the product is changed as commonly known in popcorn) the transport receptacle may be closed off at the top by a net, grid or the like. For holding such a net in place, for example, it may be impaled on guide posts arranged on the transport receptacle or clamped in place at the rim of the transport receptacle with the aid of a suitable mechanism.
The charging and discharging lock chambers are configured as pressure-tight lock chambers. It is of advantage that in this way the leakage rate of the apparatus is effectively reduced. The chambers are connected to one or more vacuum pumps or compressors via conduits each section of which may be shut off separately. When drying most of the food products, air is sucked out of the treatment chamber so that it is worked at a reduced atmospheric pressure, preferably at about 20-100 mbar. There are different food products, e. g. tea, which are dried at a pressure which is higher than atmospheric pressure. The water taken out of the food product is generally eliminated by condensation.
Preferably the volume of each charging and discharging lock chamber is very much smaller than that of the treatment chamber so that only a very minor volume of gas needs to be pumped when the transport receptacle is loaded in the charging lock chamber or unloaded from the discharging lock chamber. Thus the heat-treatment in the treatment chamber may be performed continuously or quasi-continuously. To maintain constant operating conditions the treatment chamber is preferably permanently connected to a vacuum pump or the gas reservoir.
Preferably the charging and discharging lock chambers may be shut off pressure-tight by means of doors connecting the environment or the treatment chamber, for example by means of pressure-tight slide or gate valves. For charging/discharging the product the treatment chamber is equalised in pressure each time via separate pressure equalising conduits with separately shut off means.
For conveying the transport receptacles, each of the charging and discharging lock chambers is associated to a conveyor means, conveyance being done so that the transport receptacles are transported practically with no jolting and without any excessive acceleration so as not to cause them to topple, such means making it sure to advantage that no product spillage occurs in the treatment chamber. Preferably conveyance is oriented horizontally.
Preferably the conveyor means is a conveyor belt arranged in the transport direction upstream or downstream of a conveyor belt located in the treatment chamber. To permit a more efficient transfer of the transport receptacle into and out of the treatment chamber the conveyor belt in the treatment chamber preferably features a surface roughness other than that of the conveyor belts of the charging and discharging lock chambers. Preferably the surface roughness of the belt in the treatment chamber is rougher.
In another embodiment the transport receptacle is shifted by means of a first pusher onto the floor plate of the charging lock chamber and further shifted into the treatment chamber by means of a further longitudinally movable pusher arranged in the charging lock chamber, the discharge from the apparatus being done analogously. To advantage the height of the charging and discharging lock chambers in this embodiment is very small, enabling it to be adapted to the maximum height of a transport receptacle, which helps in saving the costs of operating the system. However, any other means such as, for example, side chains, side belts, push rods and the like, for example, are possible as the conveyor means.
Arranged in the treatment chamber is at least one heating source, preferably vertically above the conveyor means and extending more or less over the full width of a transport receptacle. Preferably the product is heated or dried in the treatment chamber by means of microwave radiation. The microwave radiation is coupled into the treatment chamber and evenly distributed over an expansive zone by means of a correspondingly configured end portion of a microwave waveguide so that the treatment of the product is as uniform as possible. At the input and output end the microwave radiation is shielded practically completely from extraneous radiation by metal shields having an opening for passage of the conveyor means.
Preferably the treatment chamber is subdivided into several treatment zones by means of shielding means, such as for example metal plates having an opening for passage of the conveyor means, these treatment zones being substantially decoupled as regards the microwave radiation. Preferably simultaneous treatment of several transport receptacles or implementing several steps in the treatment are possible in the chamber to thus permit a high throughput of the system.
Preferably not a microwave source but some other heating source, such as, for example, an infrared heating source is arranged in the last segment and optionally also in the first segment of the treatment chamber so that the various steps in the method may be implemented directly in sequence in the treatment chamber under the same pressure conditions. It is furthermore of advantage that due to using microwave shielding means the doors connecting the charging or discharging lock chamber may be opened without the microwave source needing to be switched off, as long as the outermost doors of the charging or discharging lock chamber are microwave tight, this enhancing the life of the microwave source due to operation being more uniform.
In one preferred embodiment a comparatively short, intensive microwave treatment is followed by a longish resting period or dwell time, where necessary including further drying of the product under other conditions. For this purpose a further separate conveyor means is provided in the treatment chamber. To minimize the space requirement of the apparatus in accordance with the invention the transport receptacles thereof are preferably charged in a lifting conveyor system having vertical up and down guidance, for example, in a paternoster-type conveying system, the circulating speed of the lifting conveyor system being dimensioned so that the desired dwell time is maintained. Preferably, further drying is done in the treatment chamber by means of infrared heating sources arranged on the inner wall of the dwell chamber.
In another embodiment the dwell chamber is separated from the treatment chamber by a pressure-tight door so that a pressure differing to that otherwise may be built-up in the dwell chamber. So that only minor gas volumes need to be pumped in transfer of the transport receptacle, a further, charging lock chamber of small volume, as described above, is preferably provided upstream of the connecting door.
To reduce the material requirements on the conveyor means of the treatment chamber as regards microwave resistance, another preferred embodiment makes use ofxe2x80x94instead of an expansive conveyor beltxe2x80x94a conveyor means coming into contact and conveying the transport receptacle at its rim, preferably outside of the microwave zone or in zones of reduced microwave intensity. For this purpose the rim of the transport receptacle may be configured, for example, widened and rest on two narrow conveyor belts, bands or chains running along the edge of the treatment chamber which are driven at the same speed. In another embodiment the rim of the transport receptacle rests on a row of eccentrics driven in synchronism but differing in phase so that the transport receptacle is transported by a slight up and down movement in the forwards direction. It will be appreciated that other suitable means of conveyance are possible for conveying the transport receptacle.
In implementing the method in accordance with the invention the product to be treated, before entering the treatment chamber, is preferably subjected initially to a suitable predrying, for example, convective drying to a residual moisture content of down to 40% before then being subjected to the treatment as described above in the treatment chamber in which the product is exposed preferably to an intensive microwave radiation and subsequently to an optional further infrared radiation. Due to the microwave radiation the product is heated up deep-down. Preferably the intensity of the microwave radiation is set so that a voluminous cellular break-up or decomposition occurs in the case of biological materials, as is known for example in puffing popcorn from grains of maize.
After puffing, near-surface drying of the product is preferably done with the aid of infrared radiation. On leaving the treatment chamber the residual moisture content may extend as far down as to 8%, it amounting preferably to between 20% and 25%. Depending on the requirements on the finished product the residual moisture content may then be further reduced down to 2 to 6% in subsequent after-drying, for example by convective drying.
It has been discovered in extensive tests on various food products that food products treated by the method in accordance with the invention exhibit a series of highly advantageous properties. More particularly, the food products thus treated exhibit a very fast rehydration in being pre-cooked, due to the dry product remaining airy and porous due to being puffed and thus facilitate the ingress of water. The outer appearance of the treated food products is also of great advantage since the considerable shrinkage in volume caused by the drying may be compensated at least in part by the puffing. Furthermore, there is practically no change in taste because of the more gentle treatment. The method in accordance with the invention has proven to be particularly of advantage in drying banana chunks into banana chips and in their further processing into instant-type banana powder.
Preferably the method in accordance with the invention is used for drying highly-sensitive food products, such as for example banana chunks, berries or mushrooms, a further preferred use relating to drying liquid or solid or lumpy medicinal drugs.
Preferred example embodiments of the present invention will now be described by way of examples and with reference to the attached drawings in which: